Strain-gauge load cells have been widely used as a load sensor for electronic weighing scales. However, in these years, with rapid advance in electronic measurement technologies, load sensors which are more accurate than the strain-gauge load cells have been developed. Of these load sensors, different types such as a tuning fork type, a string oscillation type, a gyroscope type, and the like, have already been put into practical use.
Incidentally, as such a load sensor with a high degree of accuracy, an oscillation-type load sensor using a quartz resonator has been proposed. This load sensor takes advantage of the phenomenon that the oscillation frequency of an AT-cut quartz plate piece which is under thickness shear oscillation excited by exciting means, varies in proportion to a force applied to the quartz piece parallel to a plate face thereof. The quartz resonator has advantages such as less temperature dependency, oscillation with stable frequency, and inexpensiveness. For these reasons, the use of the quartz resonator makes it possible to attain a load sensor which is higher in accuracy and lower in cost as compared to load sensors described above, such as the tuning fork type, the string oscillation type, the gyroscope type, and the like.
FIG. 8 is a perspective view showing a constitution of basic parts of a conventional load sensor using a quartz resonator. In FIG. 8, a quartz plate resonator 300 is a quartz piece which oscillates in a thickness shear oscillation mode in the length direction. Electrodes 301, 301 are respectively affixed to both faces of the quartz resonator 300, and these electrodes 301, 301 are connected to an oscillation circuit (not shown) which oscillates in proportion to the oscillating frequency of the quartz resonator 300.
As shown in FIG. 8, grooves which are rectangular in cross-section are formed at end portions of supporting bodies 302, 302 which support the quartz resonator 300 throughout the plate widths. And, the quartz resonator 300 is retained by the supporting bodies 302, 302 in the thickness direction by fitting both of the end portions of the quartz resonator 300 into the grooves.
In the load sensor thus constructed, when a load W is applied on the quartz resonator 300 in the compressing direction, the oscillation frequency of the quartz resonator 300 changes in proportion to the load W, and then the oscillation frequency of the above-described oscillation circuit changes in proportion to the change. The load W is measured by detecting this change in the oscillation frequency.
In some cases, both of the end portions of the quartz resonator 300 and the grooves formed at the ends of the supporting bodies 302, 302 may be fixed to each other by use of adhesive or the like. In these cases, since the quartz resonator 300 remains fixed even if the supporting bodies 302, 302 move away from each other, a load W applied in the pulling direction can also be measured.
However, as described above, when both of the end portions of the quartz resonator 300 are supported by the supporting bodies 302, 302, the thickness shear oscillation of the quartz resonator 300 in the length direction is restrained, thereby causing loss of the oscillation energy. Due to this, there exists such a problem that its Q (Quality factor) as an oscillator decreases.
Furthermore, since the thickness shear oscillation of the quartz resonator 300 is transmitted to the supporting bodies 302, 302, thereby causing the surrounding mechanism to resonate, there exists such a problem that measurements can not be performed with a high degree of accuracy.